A stepper motor is a type of electric motor that moves in increments, or steps, rather than turning smoothly as a conventional electric motor does. Typically, the size of the increment is measured in degrees and can vary depending upon the application. For instance, increments can be 0.9 or 1.8 degrees, with 400 or 200 increments thus representing a full circle. Moreover, the speed of the motor is determined by the time delay between each incremental movement.
Inside the typical stepper motor, sets of coils produce magnetic fields that interact with the fields of permanent magnets. The coils are switched on and off in a specific sequence to cause the motor shaft to turn through the desired angle. The typical motor can operate in either direction (clockwise or counterclockwise). When the coils of a stepper motor receive current, the rotor shaft turns to a certain position and then stays there unless or until different coils are energized. Unlike a conventional electric motor, the stepper motor resists external torque applied to the shaft once the shaft has come to rest with current applied. This resistance is typically called holding torque.
The holding torque of a stepper motor is not constant with position but varies as the shaft moves from one full step position to the next. This variation is caused by the detent torque, which adds and subtracts from the electrically induced torque as the shaft of the motor moves. The detent torque can be approximated by a sinewave torque that repeats every full step of the motor. The magnitude of the resultant current determines the torque available from the motor.
A stepper motor is also characterized by load torque. In order to avoid loss of step, sufficient electrical current must be applied to overcome the load torque. However, the load torque can vary considerably over a pumping cycle when the motor is used within an infusion pump. Loss of step may result in a motor stall. To recover from motor stall, the motor may need to be restarted at a lower speed than that at which the stall occurred, and then accelerated to the original speed. This restart procedure requires a higher current to produce the torque. This higher torque wastes energy.
As will be appreciated by those having skill in the art, the full-step resolution of the stepper motor may be increased by applying currents to the motor coils in such proportions that the motor is positioned at some point between the full-step detent positions. Thus, in a two-phase stepper motor, energizing both coils will result in the motor being positioned halfway between the adjacent full-step positions. This is referred to as half-stepping. A full step may be further subdivided by applying a current which is the sine of the required position to one phase, and the cosine of the required position to the other phase. This is referred to as microstepping. The motor torque is a function of the vector sum of the currents applied to each phase, irrespective of the type of drive.
As indicated previously, one use of a stepper motor is to control an infusion pump such as a volumetric or peristaltic pump. Infusion pumps are used to automatically administer liquid medicants to patients. The liquid medicant is supplied from a source of medicant and delivered to the patient via a catheter or other injection device.
A common type of volumetric pump for intravenous fluids produces a peristaltic flow such as that disclosed by U.S. Pat. No. 5,842,841, incorporated herein by reference. Within this type of pump, a plastic tube leading from the bag or bottle on the drip stand to the intravenous needle (i.e. the ‘giving set’ or ‘drip-set’) passes through a special gate in which it is occluded between a row of ‘fingers’ which are moved by a cam mechanism to squeeze the closed point forward.
However, the tube is repeatedly deformed in an identical manner, thereby over the course of time destroying the elastic recovery properties of the tube so that the tube maintains a compressed aspect. This destruction of the elastic recovery properties of the tube results in the volumetric output of the pump changing markedly over time.
Today, there is a desirability of operating infusion pumps and other medical devices using a battery power source. Accordingly, conservation of power is an important feature for operating battery powered medical devices over a prolonged period of time.